Three-electrode atmospheric pressure lasers have been amply described in the literature. However, the reported configurations have proved unsatisfactory for low pressure operation. The apparatus of the instant invention is a three-electrode low pressure gas discharge tube and electronics which can be used as a laser. A discharge between an array of resistors and a highly transparent metal screen anode provides preionization of the gases employed. This causes a glow discharge to occur in the region between the anode and cathode to which is applied an unswitched dc voltage well below the breakdown potential of the entire system. When the ion buildup is sufficient, the much higher energy applied between the anode and cathode surrounding the medium in which uniform ionization is desired discharges allowing the electrical energy to be uniformly distributed in the gas. Thus, a relatively low energy preionization discharge pulse effectively initiates and controls a significantly greater energy discharge. The present invention has utility for the operation of laser amplifiers and oscillators at high repetition rates because it reduces the requirements for high voltage switch energy handling capacity, one of the technologically weak and expensive areas of laser electronics. In other words, the electronic components needed to control the plasma discharge are greatly simplified from those of the existing art. Further, it has been demonstrated that the uniform discharge characteristics occur over a wide range of laser gas mixtures when the apparatus of the instant invention is operated as a CO.sub.2 laser. In particular, stable laser action has been found for carbon dioxide-air mixtures. This remarkable result permits sealed-off operation as a CO.sub.2 laser because the discharge characteristics are unaffected by the presence of oxygen, the most important cause of unreliable operation of such devices.
A good summary of prior technology can be found in two journal articles, both describing double discharge CO.sub.2 devices using three electrodes.
1. In "Double Discharge Excitation for Atmospheric Pressure CO.sub.2 Lasers" by Albert K. LaFlamme, Rev. Sci. Instr. 41, 1578 (1970), a modified double discharge laser is discussed. By this it is meant that a single trigger device controls both the preionization and the gain medium pumping discharges. The apparatus of the instant invention also relies on one controlling voltage switch. However, and this is the critical feature of my invention, the trigger unit in our device controls only the preionization discharge. It does not have to carry any of the substantial laser discharge current that is carried by LaFlamme's trigger device. This results in substantial improvement in laser cost, reliability and pulse repetition rate.
2. In "A 300-J Multigigawatt CO.sub.2 Laser" by Martin C. Richardson, A. J. Alcock, Kurt Leopold, and Peter Burtyn, IEEE J. Quantum Electron. 9, 236 (1973), a true double discharge laser is described. A first trigger device fires the preionization discharge, while a second such device presents the main discharge until sufficient ionization has occurred in the region of anode. Here, as in Ref. 1, one trigger unit is carrying the entire discharge current. As described above, such devices are expensive, have a tendency to be unreliable and have limited rates of repetition.
The apparatus of the instant invention then, provides the uniform discharge characteristics desirable for laser oscillators and amplifiers without the use of a trigger device which "holds off" the main discharge until sufficient ionization occurs, making it an improvement, especially at lower pressures, over existing art which teaches the use of such above described triggering devices for a wide range of pressures. See e.g., Ref. 1, p. 1584. Another advantage is that the mai discharge electrodes can be charged to lower voltages than in Refs. 1 and 2 since the very uniform preionization of the instant invention permits the high energy discharge to occur at these lower voltages.